Dielectric ceramics have widely been utilized as materials for dielectric resonators and circuit boards employed in high-frequency regions such as a microwave region and a millimeter wave region.
Such high-frequency dielectric ceramics, particularly for use in dielectric resonators or dielectric filters, should have dielectric properties as follows: (1) a large relative dielectric constant (εr) to provide for required miniaturization, because the wavelength of an electromagnetic wave is reduced to 1/(εr1/2) in a dielectric material; (2) a small dielectric loss or a large Q-value, and (3) excellent temperature stability of resonance frequency or a temperature coefficient (τf) of resonance frequency of about 0 ppm/° C.
τf denotes an inclination (first-order differential coefficient) in the linear approximation of a temperature-resonance frequency curve. τf is calculated by the expression of τf=(f55−f25)/[f25×(55° C.-25° C.)], wherein f25 and f55 denote the resonance frequencies at 25° C. and 55° C., respectively.
Heretofore, many high-frequency dielectric ceramic compositions that meet the requirements described above have been proposed. Examples of the high-frequency dielectric ceramic compositions include CaTiO3—Ca{Mg1/3(Nb,Ta)2/3}O5-LnAlO3 disclosed in Patent Document 1 and CaTiO3—Ca{(Zn,Mg)1/3(Nb, Ta)2/3}O5-LnAlO3 disclosed in Patent Document 2, wherein Ln denotes a rare earth element.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-163665
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-192265